Defects, in particular in metallic components, for example, in forged products, are detected by means of ultrasonic testing. The analysis technique SAFT is known for better localization and separation of defects.
SAFT (synthetic aperture focusing technique) or synthetic aperture method refers to a method in which a B-image is obtained by computer from previously recorded, digitized, and stored A-images. A test head having the largest possible aperture angle is moved along a line. A-images are digitized at intervals and stored in a computer in this case. Depending on the test head position, echoes of a flaw occur with corresponding runtime differences. For each voxel of the test volume of a workpiece, sound runtimes to be expected are conventionally calculated from geometric relationships for each test head position and the echo amplitudes corresponding to these runtimes are searched out and totaled in the stored A-images. This results in exact runtime compensation at the location of a flaw, so that the flaw echoes are superimposed there in-phase from all test head positions and result in a correspondingly high computed amplitude. The sound intensity is therefore focused on the respective observed voxel. If the obtained amplitude value is associated with each voxel in the case of such synthetic focusing, a focused volume data set is obtained.
The inspection is performed in this case in a conventional manner, however, the HF data and the precise items of position information are recorded. During the subsequent SAFT analysis of the measurement data, amplitude totals are calculated from many time signals, the so-called A-images, in each case for small elements of the volume to be tested. Such elements of the volume to be tested are referred to as voxels. Corresponding to the respective distance between voxel and measurement point, which is the position of the test head, the amplitudes are totaled at the points in time which correspond to the respective distance of voxel to measurement point.
As a result of the aperture angle of the sound bundle emitted from the test head, defect indications are spatially blurred, so that in the sectional view, the so-called B-image, the punctiform defects become sickle-shaped indications. These sickle-shaped indications are again concentrated on punctiform indications by means of the SAFT analysis.
However, this functions only in the so-called far field; other indication shapes result in the vicinity of the test subject surface due to the test head sound field. Therefore, conventional SAFT analysis achieves poor results in the vicinity of the test surface.
Different variants of the SAFT algorithm are known, which are oriented on other problems and do not cause any improvement in the near field, however. Thus, for example, for accelerating the calculation, FT-SAFT analysis is known, which causes a significant speed advantage by using the Fourier transform for planar test surfaces. Furthermore, for testing homogeneous anisotropic materials, the HAFT-SAFT method is known, in which the direction-dependent propagation speed is taken into consideration. Instead of a spherical wave, the energy speed surfaces of a point wave depending on the respective materials are used as the basis.